In an internal combustion engine provided with a fuel injector, injected fuel is supplied to the cylinder from all intake port by spraying it into the intake air. However, as the fuel is originally in the liquid form, part of it adheres as liquid to the intake port or intake valve, and it therefore enters the cylinder in a different form to that of the fuel in the air-fuel mixture. This flow of liquid fuel, or wall flow, requires a different time to reach the cylinder from the time required by the air-fuel mixture which is in spray form, and its entry into the combustion chamber is delayed in comparison to the air-fuel mixture. When the engine is running steadily under fixed conditions, this fuel delay has no effect on the air-fuel ratio in the combustion chamber, but under transient conditions such as during acceleration or deceleration for example, the difference in the rate at which the air-fuel spray and the wall flow reach the cylinder causes the air-fuel ratio of the mixture in the cylinder to fluctuate between rich and lean, and this has an adverse effect on the composition of the engine exhaust and output.
Hence during acceleration, part of the injected fuel becomes wall flow, which causes a delay in the fuel increase with respect to the increased air flow into the cylinder, and the air-fuel ratio shifts to lean. During deceleration on the other hand, due to the wall .flow, the decrease of fuel is delayed with respect to the decrease of air flowing into the cylinder, so the air-fuel ratio shifts to rich. Moreover, not all of the air-fuel spray enters the cylinder uniformly, and part of it becomes a slow-moving flow between the cylinder injector and the cylinder. This also gives rise to some delay compared to the flow of intake air.
The delay in the fuel supply to the cylinder due to adhesion or slow movement of fuel may vary depending on for example the fuel composition, engine temperature or the structure of the intake passage. Broadly speaking, however, a short-term delay having a relatively fast time constant and small delay, and a long-term delay having a relatively slow time constant and a large delay, may be distinguished. For example, FIG. 22 shows the change of air-fuel ratio with respect to intake throttle opening during acceleration. As shown by the curve (1) in the drawing, the effect of the short-term delay appears immediately after the throttle opening is changed, and the effect of the long-term delay appears subsequently.
Therefore, by separating the flow delay due to adhesion and slow movement of fuel into a short-term delay having a relatively small (fast) time constant and a long-term delay having a relatively large (slow) time constant, and performing a wall flow correction of the fuel injection amount during transient conditions for each type of delay, high precision air-fuel ratio control can be achieved.
More specifically, the fuel injection pulse width Ti of the fuel injector is given by the following equation: EQU Ti=(Tp+Kl+Kh)*.alpha.+Ts
where: EQU Tp=basic injection pulse width EQU Kl=long-term delay flow correction EQU Kh=short-term delay flow correction EQU .alpha.=air-fuel ratio feedback control coefficient EQU Ts=ineffectual pulse width
In this equation, the two corrections Kl and Kh may both be predicted from the running conditions determined by the engine speed Ne, injector air flow (corresponding to the engine load) Q.sub.AINJ and the engine cooling water temperature Tw, and may be found by learning. During acceleration, as seen in FIG. 21, the short-term delay flow correction Kh is first learnt in the initial stage of the acceleration, the long-term delay flow correction Kl is then learnt, and a correction is performed based mainly on these learnt values. The air-fuel ratio is thereby corrected to a target air-fuel ratio as shown in FIG. 22, curve (1).
The separation of the delay flow into two types as hereintofore described and the application of separate transient corrections for the two types of flow, is disclosed for example in Tokkai Sho 63-38635 published by the Japanese Patent Office.
This correction method, however, does not properly work for an air-assisted injector, which is a fuel injector provided with a supplemental air supply for promoting atomization of fuel at the moment of fuel injection. This type of injector is known to improve the performance of an engine under steady state running conditions.
When this air-assisted injector is used, however, as the diameter of fuel spray particles becomes smaller, the delay flow time constants also vary.
Consequently, when the aforementioned correction process is applied to an air-assisted injector, an air-fuel ratio error arises as shown in the lowermost part of FIG. 22. This figure shows the relation between the throttle opening Tvo during acceleration, the fuel injection pulse width (value expressing the fuel injection amount) Ti and the air-fuel ratio before and after applying a transient correction. Curve (1) shows the air-fuel ratio in a standard injector, curve (2) shows the air-fuel ratio using an air-assisted injector wherein the fuel particles are relatively large, and curve (3) shows the air-fuel ratio using an air-assisted injector wherein the fuel particles are relatively small.
As can be seen from this figure, when the air-assisted injector is used, the air-fuel ratio tends to be too rich in the initial stage of acceleration, and to be too lean in the later stage of acceleration. This is due to the fact that the corrections Kl and Kh were set without considering differences in the atomization level of the fuel.